Gadolinite, sometimes known as ytterbite, is a silicate mineral consisting principally of the silicates of cerium, lanthanum, neodymium, yttrium, beryllium, and iron with the formula (Ce,La,Nd,Y)2FeBe2Si2O10. It is called gadolinite-(Ce) or gadolinite-(Y), depending on the prominent composing element (Y if yttrium predominates, and Ce if cerium). It may contain 35.48% yttria sub-group rare earths, 2.17% ceria earths, as much as to 11.6% BeO, and traces of thorium. It is found in Sweden, Norway, and the US (Texas and Colorado).

Gadolinite was named in 1800 for Johan Gadolin, the Finnish mineralogist-chemist who first isolated an oxide of the rare-earth element yttrium from the mineral in 1792. The rare earth gadolinium was also named for him. However, gadolinite does not contain more than trace amounts of gadolinium. When Gadolin analyzed this mineral, he missed an opportunity to discover a second element: the element he thought was aluminium (alumina) was in fact beryllium (beryllia), an element that was not officially identified until 1798.

Gadolinite and euxenite are quite abundant and are future sources of yttrium sub group rare earths. At present, these elements are recovered from monazite concentrates (after recovery of ceria sub-group metals).